Microelectronic devices in integrated circuits are manufactured by means of photolithographic techniques. Fabricating various structures, particularly electronic device structures, typically involves depositing at least one layer of at least one photosensitive material, typically known as a photoresist material, on a substrate. The photoresist material may then be patterned by exposing it to radiation of a certain wavelength or wavelengths to alter characteristics of the photoresist material. Typically, the radiation is from the ultraviolet range of wavelengths. The radiation causes desired photochemical reactions to occur within the photoresist.
The photochemical reactions alter the solubility characteristics of the photoresist, thereby allowing removal of certain portions of the photoresist. Selectively removing certain parts of the photoresist allows for the protection of certain areas of the substrate while exposing other areas. The remaining portions of the photoresist typically are utilized as masks or stencils for processing the underlying portions of the substrate.
An example of such a process is in the fabrication of semiconductor devices wherein, for example, layers are formed on a semiconductor substrate. Certain portions of the layers may be removed to form openings through the layers. The openings may allow diffusion of desired impurities through the openings into the semiconductor substrate. Other processes are known for forming devices on a substrate.
Devices such as those described above, may be formed by introducing a suitable impurity into a layer of a semiconductor to form suitably doped regions therein. In order to provide distinct P or N regions, which are necessary for the proper operation of the device, introduction of impurities should occur through only a limited portion of the substrate. Usually, this is accomplished by masking the substrate with a resist material and subsequently etching a diffusion resistant material, such as silicon dioxide or silicon nitride to a desired depth to form a protective mask to prevent diffusion of the impurities through selected areas of the substrate.
The mask in such a procedure is typically provided by forming a layer of material over the semiconductor substrate and, afterward creating a series of openings through the layer to allow the introduction of the impurities directly into the underlying surface. These openings in the mask are readily created by coating the silicon wafer with a material known as a photoresist. Photoresists can be negative photoresist or positive photoresist materials.
A negative photoresist material is one which is capable of polymerizing and being rendered insoluble upon exposure to radiation, such as UV radiation. Accordingly, when employing a negative photoresist material, the photoresist is selectively exposed to radiation, causing polymerization to occur above those regions of the substrate which are intended to be protected during a subsequent operation. The unexposed portions of the photoresist are removed by a solvent which has a minimal effect on the polymerized portion of the photoresist.
Positive photoresist material is a material that, upon exposure to radiation, is capable of being rendered soluble in a solution, such as an aqueous alkaline solution in which the unexposed resist is not soluble. Accordingly, when applying a positive photoresist material, the photoresist is selectively exposed to radiation, causing the reaction to occur above those portions of the substrate which are not intended to be protected during the subsequent processing period. The exposed portions of the photoresist are removed by an aqueous alkaline solution which has a minimal impact on the unexposed portion of the resist.
Photoresist materials may similarly be used to define other regions of electronic devices.
In an effort to increase the capability of electronic devices, the number of circuit features included on, for example, a semiconductor chip, has greatly increased. When using a process such as that described above for forming devices on, for instance, a semiconductor substrate, increasing the capability and, therefore, the number of devices on a substrate requires reducing the size of the devices or circuit features.
One way in which the size of the circuit features created on the substrate has been reduced is to employ mask or reticle structures having smaller openings. Such smaller openings expose smaller portions of the semiconductor wafer surface to the radiation, thereby creating smaller structures in the photoresist. In order to produce smaller structures in the photoresist, shorter single wavelength ultraviolet radiation may also be used in conjunction with the mask or reticle to image the photoresist in order to achieve a maximized resolution of ever decreasing feature sizes.
After forming features in the photoresist, electronic device features may be formed in or on the substrate upon which the photoresist is deposited. However, prior to forming the devices, the photoresist may be subjected to a photostabilization process.
Photostabilization typically is a post-lithography process that can maintain resist feature sidewall profiles, minimize outgassing, minimize blistering, minimize resist popping and reduce resist residue and particles. Photostabilization makes photolithographic features more rigid and more robust so as to make them more resistant to subsequent processing. Photostabilization can also reduce process delays. Photostabilization is described in U.S. Pat. No. 4,548,688 issued Oct. 22, 1985, to Matthews for Hardening of Photoresist and Apparatus, the entire disclosure of which is hereby incorporated by reference.
Photostabilization utilizes electromagnetic energy, photons, typically in the Deep UV range and heat to cure or densify a photoresist. Preferably, the heat is applied by ramping up the temperature. Typically, the photoresist is subjected to UV radiation while simultaneously being heated. The radiation and heat initiates chemical cross-linking reactions within the resist.
Some researchers have found that photostabilization of features defined with a Deep UV or chemically amplified resists causes a shrinkage in features formed in the Deep UV photoresist. Such shrinkage is considered to be very detrimental to the subsequent formation of electronic device structures in and on the semiconductor wafer upon which photoresist is deposited. The shrinkage will occur along the length, width and height of the resist features. In other words, film thickness and critical dimension will both be affected by the photostabilization process.
Horizontal shrinkage, along the length or width, can result in significant undesirable change between the feature imaged in the photoresist and the subsequently etched feature. Vertical, or height, shrinkage may result in a diminished amount of resist, which may result in inadequate protection of the underlying substrate, particularly during anisotropic etching. Many practitioners have avoided the use of Deep UV resist photostabilization processes so as to attempt to minimize the shrinkage that is known to occur during the processing of Deep UV resists.
Typically, the substrate and photoresist are subjected to processes such as etch and implant directly after the formation of features in the photoresist, such as by photolithography. This is done even though it is known that a certain amount of shrinkage occurs during processes such as etch and implant. The shrinkage is simply calculated into the size of the features created in the photoresist. However, there are inherent uncertainties in the shrinkage that occurs during, for example, etching. Typically, shrinkage occurring during etching is not reproducible, is variable, and changes with the load on the etcher.